On bounding the difference between the maximum degree and the chromatic number by a constant

TL;DR

This paper characterizes graph classes where the maximum degree is bounded by the chromatic number plus a constant, using forbidden induced subgraphs, and compares these with classes bounded by clique number.

Contribution

It provides a finite forbidden induced subgraph characterization for the class igamma_k, extending previous work on related graph classes and neighborhood properties.

Findings

Finite forbidden subgraph characterization for igamma_k.

Comparison between igamma_k and omega_k classes.

Characterization of graphs with perfect neighborhoods.

Abstract

We provide a finite forbidden induced subgraph characterization for the graph class $\varUpsilon_k$, for all $k \in \mathbb{N}_0$, which is defined as follows. A graph is in $\varUpsilon_k$ if for any induced subgraph, $\Delta \leq \chi -1 + k$ holds, where $\Delta$ is the maximum degree and $\chi$ is the chromatic number of the subgraph. We compare these results with those given in [O. Schaudt, V. Weil, On bounding the difference between the maximum degree and the clique number, Graphs and Combinatorics 31(5), 1689-1702 (2015). DOI: 10.1007/s00373-014-1468-3], where we studied the graph class $\varOmega_k$, for $k \in \mathbb{N}_0$, whose graphs are such that for any induced subgraph, $\Delta \leq \omega -1 + k$ holds, where $\omega$ denotes the clique number of a graph. In particular, we give a characterization in terms of $\varOmega_k$ and $\varUpsilon_k$ of those graphs where the neighborhood of every vertex is perfect.